This Research Scientist Award Application continues a line of research aimed at gaining a better understanding of the role of early separation in development. The proposed experiments focus on the acute separation response of infant rats. It has become evident that this behavior is a promising model for studying the development of anxiety. During the last grant period, we have defined the major components of a relatively simple attachment system in the infant rat. These components consist of the ultrasonic vocalization (USV) or separation distress call, its elicitation by isolation, cold and novelty, and its regulation by contact with familiar social companions and by decreased core temperature. The first set of studies will investigate the neuromuscular apparatus by which the pup produces USVs, the neural pathways by which it responds to isolation or contact experience, and the central opiate receptor mechanisms for these responses. The second set of studies is aimed at understanding the behavioral and physiological processes involved when response intensity is regulated by contact with companions in the short term, and the mother-infant interaction over a longer time period. Thirdly, the genetic basis for individual levels of infant distress vocalization will be explored through a selective breeding approach and we will test the hypothesis that long term developmental continuity exists between infant USV responses and measures of anxiety in adulthood. Collaborative studies consist of co-sponsoring two RSDA awardees, and serving as co-investigator in the independent research of a younger colleague on an animal model for Sudden Infant Death Syndrome, a project deriving from a discovery made in a prior RSA grant period.